The present invention relates generally to electronic systems and more particularly to an electromagnetic interference shield for overmolded packaging of an electronic assembly.
Circuit boards with semiconductor devices such as flip chips must often be protected from the environment in which the board is employed. A widely practiced method is to enclose such circuit boards in an assembly that includes a pair of case halves that must be assembled together by hand to form an enclosure that supports the circuit board within. Connectors secured to one of the case halves provide for electrical interconnection to the circuit board. Sealing elements are also typically required to exclude moisture from the enclosure. Finally, fasteners are required to secure the assembly together. Such assembly processes are labor intensive, and the resulting package must be tested for leakage to ensure the package was properly assembled.
To simplify the design, an overmolded electronic assembly that is compatible with automated assembly methods has been used. The overmolded assembly, typically comprised of plastic or epoxy type material, includes a heat conductive member, such as a heat sinking backplate, in thermal contact with one or more of the circuit devices mounted to the circuit board. The overmolded body encloses the circuit board and the circuit device with the heat conductive member, such that the overmolded body and heat-conductive member form a moisture-impermeable seal around the circuit board and circuit device. The overmolded body has a connector housing integrally formed in its outer surface. The overmolded body is non-metallic and is typically formed of a plastic or epoxy-type material and provides a secure environment against vibration and shock. One example of an overmolded electronic assembly is described in U.S. Pat. No. 6,180,045 to Brandenburg et al., which is herein incorporated by reference.
One problem with overmolded electronic assemblies is that the non-metallic topside of the assembly cannot shield electromagnetic interference (xe2x80x9cEMIxe2x80x9d) emissions as can typical sheet metal or die cast metallic housings.
From the above, it can be appreciated that an electronic assembly that combines the simplified processing and improved moisture and vibration resistance of an overmolded body assembly with improved EMI emissions resistance would be highly desirous.
According to the present invention, there is provided an overmolded electronic assembly having a thin metal shield, in the form of a film or foil, coupled to a top portion of the overmolded module. The thin metal shield effectively reduces the amount of EMI emissions from penetrating within the assembly to the circuit board without substantially increasing the cost of the unit. It also prevents EMI radiated emissions generated inside the assembly from escaping outside of the enclosure. Thus, an electronic assembly having improved vibration, moisture, and EMI emission and radiation resistance is achieved as compared with traditional overmolded or metal assemblies. Further, because the metal shield can be formed on the electronic assembly in one process step, a substantial savings in time and cost for the manufacturing process can be realized.
The method for manufacturing the electronic assembly can be performed in three distinct ways. In one method, the metal shield is preformed as a film to a desired shape and placed in a mold cavity just prior to molding. A vacuum holds the metal film preform in place during the molding process. The metal film is then adhered to a top surface of the overmolded body of the overmolded electronic assembly. In another method, the roll of metal foil is incorporated into a film assisted molding equipment (xe2x80x9cFAMExe2x80x9d) mold press. A modified FAME process is used to incorporate the metal film within the overmolded body of the overmolded electronic assembly. In a third method, the overmolded assembly is first formed and then a thermal or kinetic spray coating of metal film is applied to the outside of the overmolded assembly.
Other objects and advantages of the present invention will become apparent upon considering the following detailed description and appended claims, and upon reference to the accompanying drawings.